Oil Compensation In A Refrigeration Circuit

ABSTRACT

The invention relates to a refrigeration circuit with at least two compressors, each compressor comprising at least one lubrication point and at least one oil pump having an outlet for providing oil to said lubrication point; wherein the outlet of the oil pump of at least one of the compressors is fluidly connected to the lubrication point of at least one different compressor.

Refrigeration circuits for circulating a refrigerant and comprising in flow direction of the refrigerant: a heat rejecting heat exchanger, a receiver, an expansion device, an evaporator, and a compressor are widely known and used for refrigeration purposes. Some embodiments include a couple of compressors connected in parallel in order to increase the performance of the refrigeration circuit. Using a couple of compressors in parallel increases not only the performance but also the flexibility of the circuit as individual compressors can be switched on and off in order to adjust the performance provided by the compressor assembly. It further increases the reliability of the circuit as the circuit can be operated even if one of the compressors is broken or not working due to maintenance.

Usually the compressors are lubricated by oil circulating through the compressor and flowing by its moving parts. When at least two compressor of this type are connected in parallel, there exists the problem that the oil level of the individual compressors will change during operation due to oil which is carried over to the refrigerant and circulates through the refrigeration cycle together with the refrigerant. This transfer of oil between the compressors may result in one or more of the compressors running completely out of oil which may cause severe damage of the respective compressor(s).

U.S. Pat. No. 5,586,450 A discloses a refrigeration circuit comprising two compressors. Each compressor has an oil pump with excess capacity relative to lubrication requirements of the respective compressor. The excess capacity is diverted to a common line which connects to each of the compressors in the circuit through a float valve. Each float valve is operated responsive to the oil level in the associated compressor such that oil diverted as excess capacity is supplied to any compressor in the circuit short of oil.

It would be beneficial to provide means for keeping the oil level in a plurality of individual compressors working in parallel constant and to provide a method for operating a refrigeration circuit comprising a plurality of compressors which keeps the oil level in the individual compressors constant during operation.

Exemplary embodiments of the invention include a refrigeration circuit having at least two compressors. Each compressor includes at least one lubrication point, i.e. a point which is to be supplied with oil when the compressor is operating, and at least one oil pump having an outlet for providing oil to said lubrication point. The outlet of the oil pump of at least one of the compressors is fluidly connected to the lubrication point of at least one other compressor in order to allow said oil pump to provide oil to the lubrication point of said other compressor.

Exemplary embodiments of the invention further include a method of operating a refrigeration circuit with at least two compressors, wherein each of the compressors has at least one lubrication point and at least one oil pump with at least one outlet. The method includes the step of supplying oil from the outlet of one of the oil pumps to the lubrication point of at least one other compressor.

Embodiments of the invention are described in greater detail below with reference to the attached figures, wherein:

FIG. 1 shows a schematic view of a refrigeration circuit in accordance with a first embodiment the invention;

FIG. 2 shows a schematic view of a refrigeration circuit in accordance with a second embodiment the invention;

FIG. 3 shows a schematic view of a refrigeration circuit in accordance with a third embodiment the invention;

FIG. 1 shows a schematic view of a refrigeration circuit which is configured for circulating a refrigerant in counter-clockwise direction as indicated by the arrow A. The refrigeration circuit comprises in flow direction of the refrigerant a heat rejecting heat exchanger 2, which is configured for cooling the refrigerant, a receiver 4, which is configured for storing the refrigerant, a couple of expansion devices 6 a, 6 b, which are configured for expanding the circulating refrigerant, i. e. reducing the pressure of the refrigerant, and a two evaporators 8 a, 8 b, which are arranged downstream of the expansion devices 6 a, 6 b, respectively, and which are configured for heating and evaporating the expanded refrigerant.

While in the exemplary embodiment shown in FIG. 1 two expansion devices 6 a, 6 b and two evaporators 8 a, 8 b are connected in parallel, it is evident to the skilled person that any number of expansion devices 6 a, 6 b and evaporators 8 a, 8 b may be connected in parallel in order to fulfill individual needs. In particular, in its most simple form the refrigeration circuit may comprise only one expansion device 6 a and only one evaporator 8 a.

The refrigeration circuit further includes an assembly 10 of compressors 10 a, 10 b, 10 c which are connected in parallel with their inlets and outlets in order to suck refrigerant leaving the evaporators 8 a, 8 b, compressing the refrigerant and dispensing the compressed refrigerant to the heat rejecting heat exchanger 2.

In the exemplary embodiment shown in FIG. 1 the assembly 10 includes three compressors 10 a, 10 b, 10 c connected in parallel. However, it is evident to the skilled person that any desired number of compressors 10 a, 10 b, 10 c may be used.

Each of the compressors 10 a, 10 b, 10 c comprises an oil sump 20 a, 20 b, 20 c for collecting oil circulating through the compressor 10 a, 10 b, 10 c during operation in order to lubricate the moving parts of the compressors 10 a, 10 b, 10 c.

The oil sumps 20 a, 20 b, 20 c of the compressor 10 a, 10 b, 10 c are connected to each other by means of an oil sump compensation line 22 allowing oil to flow from each of the oil sumps 20 a, 20 b, 20 c to the oil sump(s) 20 a, 20 b, 20 c of another compressor 10 a, 10 b, 10 c, in order to perform oil compensation between the oil sumps 20 a, 20 b, 20 c of the compressors 10 a, 10 b, 10 c. Since no oil pumps are involved in this type of oil compensation, this type of oil compensation may be considered as passive oil compensation.

In particular, if the level of oil in one of the oil sumps 20 a, 20 b, 20 c exceeds the level of oil in the oil sump 20 a, 20 b, 20 c of another compressor, this results in a pressure difference between the oil sumps 20 a, 20 b, 20 c of the respective compressors 10 a, 10 b, 10 c, which will generate a flow of oil from the oil sump 20 a, 20 b, 20 c comprising more oil to the oil sump 20 a, 20 b, 20 c comprising less oil.

Each of the compressors 10 a, 10 b, 10 c further includes an oil pump 12 a, 12 b, 12 c configured for delivering oil from the respective oil sump 20 a, 20 b, 20 c to the lubrication points, i. e. points which need to be lubricated during the operation of the compressor 10 a, 10 b, 10 c.

Providing an array 10 of compressors 10 a, 10 b, 10 c with oil pumps 12 a, 12 b, 12 c and an oil sump compensation line 22 connecting the oil sumps 20 a, 20 b, 20 c of the compressors is known in the state of the art.

A refrigeration circuit according to the invention further includes a common oil compensation line 18 fluidly connecting the outlets of the oil pumps 12 a, 12 b, 12 c of the compressors 10 a, 10 b, 10 c with each other and with the lubrication points of the other compressors 10 a, 10 b, 10 c.

Said common oil compensation line 18 allows every oil pump 12 a, 12 b, 12 c of each of the compressors 10 a, 10 b, 10 c to deliver oil to the lubrication points of each of the plurality of compressors 10 a, 10 b, 10 c.

Thus, even if one of the oil sumps 20 a, 20 b, 20 c should be drained during the operation of the compressors 10 a, 10 b, 10 c the lubrication points of the drained compressor 10 a, 10 b, 10 c will be lubricated by oil delivered by at least one oil pump 12 a, 12 b, 12 c of another compressor 10 a, 10 b, 10 c. The oil delivered to said compressor 10 a, 10 b, 10 c via the common oil compensation line 18 will flow to the drained compressor's oil sump 20 a, 20 b, 20 c after it has passed and lubricated the moving parts of said compressor 10 a, 10 b, 10 c.

Thus, a drained oil sump 20 a, 20 b, 20 c of any of the compressors 10 a, 10 b, 10 c will be refilled by transferring oil from at least one of the other compressors 10 a, 10 b, 10 c via the common oil compensation line 18 to each of the compressors 10 a, 10 b, 10 c, which has run out of oil.

Damage of the compressors 10 a, 10 b, 10 c due to lack of lubricating oil is therefore reliably prevented. Expensive devices or sophisticated control algorithms are no longer needed in order to avoid an insufficient oil distribution.

Furthermore, even if one of the oil pumps 12 a, 12 b, 12 c is broken or stopped due to maintenance, a sufficient oil supply to each of the compressors 10 a, 10 b, 10 c is reliably provided by the oil pumps 12 a, 12 b, 12 c of the additional compressors 10 a, 10 b, 10 c. This increases the reliability of the refrigeration circuit even further.

In the exemplary embodiment shown in FIG. 1 switchable valves 14 a, 14 b, 14 c are arranged between the outlets of the oil pumps 12 a, 12 b, 12 c of each of the compressors 10 a, 10 b, 10 c and the common oil compensation line 18. If one of the compressors 10 a, 10 b, 10 c is switched off for maintenance or due to low load of the refrigeration circuit, the corresponding switchable valve 14 a, 14 b, 14 c may be closed in order to avoid an unnecessary supply of oil to said non-working compressor 10 a, 10 b, 10 c.

By selectively opening and closing the respective switchable valves 14 a, 14 b, 14 c the oil compensation between the compressors 10 a, 10 b, 10 c can be selectively controlled in order to equalize the oil levels in the plurality of oil sumps 20 a, 20 b, 20 c.

In an embodiment of the invention at least one of the compressors 10 a, 10 b, 10 c may include a variable speed drive (VSD) 16 which allows to control the speed and the performance of the respective compressor 10 a. This allows to adjust the performance of the assembly 10 of compressors 10 a, 10 b, 10 c not only by switching on and off individual compressors 10 a, 10 b, 10 c but additionally by modifying the speed of at least one of the compressors 10 a.

When a common oil compensation line 18 is used in connection with a compressor 10 a comprising a variable speed drive 16 the lubrication of the VSD driven compressor 10 a will be safer and the oil carry over rate (OCR) will be reduced.

The skilled person will easily understand that the receiver 4 is an optional feature of the refrigeration circuit and that the invention may also be applied to refrigeration circuits without a receiver 4.

A refrigeration circuit having a refrigerator 4 may include a flash-gas line extending from the receiver 4 to the inlet of the compressor(s) 8 a, 8 b and being configure for tapping flash gas from the receiver 4 to the compressor(s) 8 a, 8 b bypassing the expansion device(s) 6 a, 6 b and the evaporator(s) 8 a, 8 b. A flash-gas line helps to improve the performance as well as the efficiency of the refrigeration circuit.

In a second embodiment shown in FIG. 2 the refrigeration circuit is a two-stage expansion refrigeration circuit comprising a first, high pressure expansion valve 3 arranged between the outlet of the heat-rejecting heat-exchanger 2 and the receiver 4 and a second, low-pressure expansion devices 6 a, 6 b arranged in flow direction between the outlet of the receiver and the inlet of the evaporator(s) 8 a, 8 b.

In such a two-stage expansion refrigeration circuit high pressure refrigerant leaving the heat-rejecting heat-exchanger 2 is partially expanded by the high pressure expansion valve 3 to medium pressure. The refrigerant is stored in the receiver 10 at said medium pressure.

The refrigerant taken from the receiver 10 is expanded from medium-pressure to low pressure by at least one low-pressure expansion device 6 a, 6 b before entering the evaporator(s) 8 a, 8 b.

The two-stage expansion as performed in two-stage expansion refrigeration improves the performance and efficiency of the refrigeration circuit, in particular if CO₂ is used as a refrigerant. CO₂ provides a very efficient refrigerant.

FIG. 3 shows a further embodiment, which is similar to the first embodiment shown in FIG. 1. In said second embodiment, however, the oil sumps 20 a, 20 b, 20 c of the compressors 10 a, 10 b, 10 c are not connected by an oil sump compensation line 22 as in the first embodiment shown in FIG. 1.

In this second embodiment oil compensation is performed completely via the common oil compensation line 18 fluidly connecting the outlets of the oil pumps 12 a, 12 b, 12 c of the individual compressors 10 a, 10 b, 10 c. I. e. oil compensation between the individual oil sumps 20 a, 20 b, 20 c is performed by pumping oil from at least one of the oil sumps 20 a, 20 b, 20 c to the lubrication points of at least one different compressor 10 a, 10 b, 10 c by the oil pumps 12 a, 12 b, 12 c and the common oil compensation line 18. The oil will then flow from said lubrication points to the oil sump 20 a, 20 b, 20 c of the respective compressor 10 a, 10 b, 10 c refilling said oil sump 20 a, 20 b, 20 c.

In said second embodiment the space needed for the oil sump compensation line 22 and the costs for providing and maintaining said oil sump compensation line 22 can be saved. Thus, the costs for a refrigeration circuit according to the second embodiment are reduced with respect to a refrigeration circuit according to the first embodiment.

Exemplary embodiments of the invention as described above allow for an effective and reliable oil compensation between the compressors. In particular it provides a reliable oil supply to all the compressors even if one of the oil pumps is not working or one of the oils sumps has run out of oil. It therefore improves the reliability of a refrigeration circuit comprising a plurality of compressors.

It is possible that the outlet of the oil pump of each compressor is fluidly connected to the lubrication points of at least one additional compressor. This ensures a reliable oil supply to the lubrications points of each compressor.

It is also possible that every lubrication point of each of the compressors is fluidly connected to the oil pump of at least one different compressor. This ensures a reliable oil supply to the lubrication points of all the compressors.

The outlets of the oil pumps of the compressors may be fluidly connected by a common oil compensation line. A common oil compensation line allows to perform oil compensation between the outlets of the oil pumps of all the compressors at low costs.

It is possible that said common line is fluidly connected to the lubrication points of all compressors. This provides reliable oil supply to the lubrication points of all compressors and therefore increases the reliability of the refrigeration circuit.

The compressors may include oil sumps and oil pumps which are configured to deliver oil from the respective oil sump to the lubrication points of the respective compressor. Oil sumps provide effective means for collecting and storing an amount of oil within the compressor and providing said oil to an oil pump for delivery to respective lubrication points.

The oil sumps of a couple of compressors may be fluidly connected to each other by an oil sump compensation line. Fluidly connecting the oil sumps of a couple of compressors allows easy oil compensation between the compressors by oil flowing directly from one oil sump to the oil sump of another compressor due to different oil pressures in the different oil sumps, the different oil pressures being caused by different oil levels in the different compressors.

In an embodiment the oil sumps of the compressors are not fluidly connected to each other. This saves the costs for the oil sump compensation line fluidly connecting the oil sumps of the compressors. In this case oil compensation is performed only via the fluidly connected outlets of the oil pumps as described above.

At least one of the compressors may include a variable speed drive. A compressor comprising a variable speed drive allows for flexible adjustment of the performance of said compressor. It also allows a finer adjustment of the performance of a set of compressors than the on-/off-switching of one or more of said compressors.

A switchable valve may be arranged between the outlet of the oil pump of at least one of the compressors and at least one of the lubrication points. In particular this switchable valve may be a solenoid valve. A switchable valve allows to fluidly separate the respective compressor from the other compressors, in particular if the respective compressor is not running. Separating a non-working compressor from the other compressors allows to avoid an undesirable flow of oil to and from a non-working compressor.

A switchable valve may also be arranged between the outlet of the oil pump of at least one of the compressors and the common oil compensation line described before in order to allow to avoid an undesirable flow of oil between a non-working compressor and the common oil compensation line by closing the switchable valve corresponding to the non-working compressor.

During operation of a refrigeration circuit oil from one of the oil pumps may be supplied to the lubrication points of all the compressors. This ensures that all compressors are reliably supplied with lubricating oil.

The operation may also include to supply oil from the oil pumps of all the compressors to the lubrication points of all the compressors. This provides a very efficient and reliable lubrication of the all the compressors.

The oil may be delivered through a common oil compensation line fluidly connecting the outlets of the oil pumps of the compressors. This provides a very efficient means for delivering the oil.

Operating a refrigeration circuit may include actuating at least one switchable valve arranged between the outlet of the oil pump of one of the compressors and the common line. This allows to avoid an undesirable oil exchange between the common line and a compressor which is not working.

The operation may also include to deliver oil from an oil sump formed in at least one of the compressors. Delivering oil from an oil sump, which is formed in at least one of the compressors, provides an efficient way of delivering oil.

Operating a refrigeration circuit may further include regulating the speed of at least one of the compressors. This allows to adjust the performance of the compressor or a group of compressors efficiently to the load of the refrigeration circuit.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalence may be substituted for an essential scope thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention is not limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. Refrigeration circuit with at least two compressors, each compressor comprising at least one lubrication point and at least one oil pump having an outlet for providing oil to said lubrication point; wherein the outlet of the oil pump of at least one of the compressors is fluidly connected to the lubrication point of at least one different compressor.
 2. Refrigeration circuit according to claim 1, wherein the outlet of the oil pump of each compressor is fluidly connected to the lubrication point of at least one different compressor.
 3. Refrigeration circuit according to claim 1, wherein the lubrication point of each compressor is fluidly connected to the oil pump of at least one different compressor.
 4. Refrigeration circuit according to claim 1, wherein the outlets of the oil pumps of all compressors are fluidly connected by a common oil compensation line.
 5. Refrigeration circuit according to claim 4, wherein the common oil compensation line is fluidly connected to the lubrication points of all the compressors.
 6. Refrigeration circuit according to claim 1, wherein the compressors comprise oil sumps and the oil pumps are configured for delivering oil from the oil sumps of the compressors to the lubrication points.
 7. Refrigeration circuit according to claim 6, wherein the oil sumps of the compressors are fluidly connected.
 8. Refrigeration circuit according to claim 6, wherein the oil sumps of the compressors are not fluidly connected.
 9. Refrigeration circuit according to claim 1, wherein at least one of the compressors comprises a variable speed drive.
 10. Refrigeration circuit according to claim 1, wherein there is at least one switchable valve arranged between the outlet of the oil pump of at least one of the compressors and at least one of the lubrications points.
 11. Refrigeration circuit according to claim 4, wherein there is at least one switchable valve arranged between the outlet of the oil pump of at least one of the compressors and the common oil compensation line.
 12. Refrigeration circuit according to claim 10, wherein the at least one switchable valve is a solenoid valve.
 13. Method of operating a refrigeration circuit comprising a least two compressors each of the compressors having at least one lubrication point and at least one oil pump with at least one outlet, wherein the method comprises supplying oil from the outlet of one of the oil pumps to the lubrication point of at least one other compressor.
 14. Method of claim 13 further comprising to supply oil from one of the oil pumps to the lubrication points of all compressors.
 15. Method of claim 13 further comprising to supply oil from the oil pumps of all compressors to the lubrications points of all compressors.
 16. Method of claim 13 further comprising delivering the oil through a common oil compensation line fluidly connecting the outlets of the oil pumps of the compressors.
 17. Method of claim 16 further comprising actuating at least one switchable valve arranged between the outlet of the oil pump of one of the compressors and the common oil compensation line.
 18. Method of claim 13 further comprising delivering oil from an oil sump formed in at least one of the compressors.
 19. Method of claim 13 further comprising regulating the speed of at least one of the compressors. 